A capacitance sensor used in a touch sensor, by detecting a change in a capacitance among a plurality of electrodes with a detection circuit, can detect a touch of an object to be detected such as a human body or the like.
FIG. 18 illustrates one example of a detection principle of the capacitance sensor. As detection elements, two electrodes 1a and 1b are spaced a certain distance apart on a substantially identical plane, and the electrodes 1a and 1b are covered with an insulator 2. For the electrodes 1a and 1b, as illustrated in FIG. 19, for example, the electrode 1b may be laid out around the electrode 1a, and furthermore, a guard member 3 may be laid out around the electrode 1b, keeping the electrodes in electrical isolation from another electrode. Then, both electrodes 1a and 1b are covered with the insulator.
Then, as illustrated in FIG. 18, as a capacitance Cs1 between the electrodes 1a and 1b or capacitances Cs2 and Cs3 between each electrode 1a and 1b and a ground GND change when the isolator insulator 2 on the electrodes 1a and 1b is touched with a finger, an existence of a touch with a finger may be detected by detecting the change in the capacitances with a detection circuit.
A capacitance sensor illustrated in FIG. 20 includes an electrode 6 disposed in a mounting hole 5 in an insulator 4. The electrode 6 is covered with a flexible conductive material 7 spaced a certain distance away therefrom. When the conductive material 7 is pressed and flexed, a distance between the conductive material 7 and the electrode 6 is reduced, a capacitance between the conductive material 7 and the electrode 6 changes, and, by detecting the change in the capacitance by a detection circuit, an existence of a touch with a finger is detected.
In the kind of capacitance sensor illustrated in FIG. 18, by adopting a configuration wherein a change in the capacitance Cs1 is detected by the detection circuit, a change in the capacitance Cs1 due to a touch with a finger is comparatively large and relatively unsusceptible to a parasitic capacitance. Therefore, it is possible to detect the existence of a touch with a high accuracy. However, since a detection signal similar to a detection signal obtained from a touch with a finger may be transmitted even when a water droplet or other conductive material adheres to the insulator 2, problems such as malfunctions can occur.
Also, in the event of adopting a configuration wherein a change in the capacitance Cs2 between the electrode 1a and the ground GND, or the capacitance Cs3 between the electrode 1b and the ground GND, is detected by the detection circuit, such a change in capacitance is susceptible to a parasitic capacitance between each electrode 1a and 1b and the ground GND, thus reducing detection accuracy.
Therefore, when a configuration with a function of canceling a parasitic capacitance every time is provided in the detection circuit, and with improved detection accuracy by a calibration operation is adopted, there is a problem in that the scale of the detection circuit increases.
As a disclosed technology, a sensor circuit which detects a change in a capacitance between two electrodes is disposed in JP-A-2000-65514.
Also, a humidity sensor which detects humidity by detecting a capacitance changing in accordance with a change in humidity is disclosed in JP-A-2006-58084.